A panoramic display is generally considered one in which the horizontal dimension is much greater than the vertical dimension, such that the display horizontally wraps around the viewing point, at least partially. Panoramic visual display systems are desirable in a number of applications. One particular application needing wide fields of view is flight simulation and training. Heretofore, panoramic flight simulator visual display systems have typically used multiple projectors, with their separate images edge-blended in a mosaic to create an apparently continuous wide field of view. Anywhere from three to ten or more individual projectors are commonly used, and multiple projector displays with 64 or more separate projectors have been proposed to meet the need for high resolution imagery over a very wide field of view.
Two types of such displays have frequently been used in simulation and training. The first type, shown in FIG. 13, comprises a panoramic real image display system 100, wherein a projected real image 102 is viewed by an observer from a viewing point 106 near the center of a curved front projection screen 108, in this case a dome. In such a system, multiple projectors 110 are typically located above and/or behind the viewer, preferably outside of view. For very large fields of view, a nearly complete dome display screen is used, as shown. However, it becomes increasingly difficult with increasing field of view to place projectors in locations that provide uniform illumination and resolution, yet are still hidden from view. In addition, current methods for blending the edges of images from adjacent projectors are not completely effective at hiding the seams between images, and color matching of adjacent images from multiple projectors is very difficult. The common result is composite images with noticeable seams and inconsistent coloring from one portion of the image to another.
Another frequently used display type is a wide angle virtual display 112, shown in FIGS. 14 and 15. This type of display is frequently used for civil aviation simulators. In this type of display, an observer views a concave mirror 114 which provides a reflection of an image that has been projected onto a back-projection screen 116. The object for the collimating mirror is a real image projected by multiple projectors 118. In this system both the projectors and the eyepoint 120 (inside the simulator cockpit 122) are offset from the center of curvature 126 of the screen, and the center of curvature 124 of the mirror.
The geometry shown in FIGS. 14 and 15 creates problems for projectors in general and for panoramic displays with multiple projectors in particular. As illustrated in FIG. 15, there is a single point 130 in space that is preferred by an optical projection system for directing the greatest amount of light toward the viewing location 120. Ideally, all of the light would be projected downward and around from this point to all areas of the projection screen 116. However, it is impossible to locate multiple projectors at a single point in space. Additionally, the low f-number lenses required for CRT projectors normally used do not have sufficient depth of focus to allow positioning at the preferred location 130. Also, large projectors in these positions are problematic for training applications that use motion bases. The large mass of multiple projectors placed high above the screen creates an unacceptably large moment of inertia for simulators or other applications incorporating motion systems. For these reasons, current wide-angle virtual systems, such as that shown in FIG. 14, have projectors located in a compromised position below and behind the ideal location.
A laser projector can have sufficient depth of focus to project from the ideal point well above the screen, but it must be scanned with a significant downward tilt relative to the vertical axis of the screen. The type of scanner that has been used with laser projectors in the past is not capable of the wide angle, tilted scan required for both a wide angle virtual display and a panoramic real image display.
Panoramic scanners have been developed for linear detector arrays. In prior systems, wide angle scanning of a linear (or two dimensional) array of sensors or light modulators is usually done with a mirror or prism at a 45 degree angle to an axis of rotation that is perpendicular to the axis (or plane) of the sensor. Since such an arrangement causes the projected or sensed image to roll through 360 degrees for each revolution of the scanning element as it is rotated, such scanners normally incorporate a doubly reflecting mirror system or prism rotating at half the scanner rate to restore the image to the proper orientation. These systems provide a full 360 degree scan, but require an image derotator turning continuously at one half the scan rate. Since such systems require dynamic synchronization of two elements rotating at different rates while maintaining precise optical alignment through multiple reflections, they are expensive to produce, suffer from accuracy problems, and are heavy and difficult to balance. Consequently, they are seldom used for high speed scanning systems.
In the past, laser projectors have only had the capability to scan a single beam of light over a narrow angle while modulating it one pixel at a time to create a projected raster. There are many types of laser projectors that scan a single beam in a raster fashion, such as using a rotating polygon wheel for a fast axis, and a galvanometer for a slow axis. This type of projector is limited in image size vs. projection distance, and therefore creates a narrow image. These types of projectors sometimes use a magnifying lens to shorten the throw distance. This method is unsuitable for a panoramic display because it magnifies the pixels in both the horizontal and vertical directions. Additionally, bandwidth limitations have hindered the development of a high resolution panoramic display from a single projector modulating a single laser beam.
One method of scanning a linear array to form an image uses a grating light valve (GLV) to project a linear array over a narrow field of view on a flat screen. Unfortunately, mere use of a GLV does not address the problem of creating a panoramic display on a curved screen. Prior systems for wide angle image projection also include head-mounted laser projectors incorporating a line scanner. This type of system scans a 1-dimensional image over a relatively limited field of view. Since there is no offset, the scanner must be mounted at the viewing position (e.g. on a helmet).